1. Field of the Invention
The present invention refers to a DC/DC converter.
2. Description of the Related Art
Converters of direct current into direct current, known as DC/DC converters, are generally known in the prior art. Said converters are of the switching type and they supply voltage regulated at a load by means of switching the current flow in an inductor coupled to the load.
A buck converter is a type of DC/DC converter that uses power transistors, such as MOS, to control the current flow in the output inductor. A first transistor called “high side” couples the inductor with the supply voltage while a second transistor called “low side” connects the inductor to ground. A drive circuit in PWM (Pulse Width Modulation) mode is used to drive the first and the second transistors in alternated mode. A diode can be used in place of the low side transistor.
To regulate the performance of a DC/DC converter the current sent to the load is measured. This information enables the load to be protected against overload currents, ensures control during the variations of the load and permits, in the case of multiphase converters, the balancing of the currents of the single phases.
A technique for measuring this current consists in using sense resistors arranged in series with the output inductor; the voltage from which the current is obtained is measured at the terminals of the sense resistance. The resistance should have a certain value to supply an accurate measurement; nevertheless a value of the sense resistance that is too high would lower the efficiency of the converter and would lead to problems of dissipation of heat due to the resistance.
Alternatively, the sense resistance is placed in series with one of the transistors of the converter with a terminal connected to the supply voltage or to ground. In this manner higher efficiency is obtained given that the average current that flows in the sense resistor is less than the current that flows on the load. In high-frequency applications the disturbances caused by the switching of the transistors have to be eliminated and this implies the introduction of a more complex control circuitry.
Another technique to measure the current on the load consists of positioning a series of a resistance and a capacitor in parallel with the inductor. The current in the inductor has one alternating and one direct component; the alternating component depends on the value of the inductor and on the resistance of the inductor. By suitably tuning the series of the capacitor and the resistance so that their product is equal to the ratio of the value of the inductor on the resistance of the inductor, the current on the inductor can be measured by measuring the voltage on the capacitor, without introducing losses. This technique presents some difficulties linked to tuning the series of the resistance and the capacitor and to the low immunity to noise that the voltage measured on the capacitor presents.